1. Field of the Invention
The present invention relates to a laser power adjustment method and an optical recording and reproduction apparatus, in particular, a technique suited for adjustment of recording laser power based on a γ value of a recording medium.
2. Description of the Related Art
Nowadays, in addition to reproduction-only type media such as CD-ROMs and DVD-ROMs, various optical recording media are commercialized and are widely used, examples of which are rewritable type media, such as CD-RWs and DVD-RWs, and write once type media such as CD-Rs, DVD-Rs, and DVD+Rs. Optimum laser power for recording and reproduction onto and from these optical recording media varies from media type to media type. In addition, even in the case of the same type media, the optimum laser power varies from medium to medium depending on, for instance, manufacturers of the media. Therefore, a drive apparatus that performs recording and reproduction onto and from the optical recording media is provided with a configuration for adjusting laser power for recording and reproduction to optimum values in accordance with the types, manufacturers, and the like of the media.
In standard specifications of the CD-RWs and DVD+RWs among the optical recording media, usage of a recording laser power adjustment method (γ method) that is based on γ values is described. With the adjustment method, recording laser power is adjusted to optimum values based on the modulation factors of reflection light intensities detected at the time of test writing and target γ values set for media in advance.
Also, in the standard specification of the DVD+RWs, it is in particular recommended to use a linear fit method that is one kind of the γ method. With the linear fit method, optimum laser power Pw is set for each medium in the manner described below. First, an integrated value Sn is obtained from each kind of recording laser power Pwn set for test writing and the modulation factor m(Pwn) of a reflection light intensity obtained through the test writing at the laser power Pwn using an expression “Sn=m(Pwn)×Pwn”. Next, laser power Pwth1 is obtained at which the modulation factor becomes zero when relational characteristics of Sn and Pwn is linearly approximated. Then, the optimum laser power Pw is obtained from Pwth1 and the target γ value of the medium using an expression “Pw=Pwth×(1+1/γ)”. This linear fit method is based on the assumption that a linear approximation expression is established for the relational characteristics of Sn and Pwn.
As a result of an actual measurement of the relational characteristics of Sn and Pwn conducted by the inventors of the present invention, however, it was found that respective measurement points are not arranged in a straight line manner and meander to some extent. Accordingly, it was found that it is impossible to precisely perform the linear approximation only with around two or three Sn and it is impossible to obtain accurate laser power unless the test writing and the computation of Sn are repeated a considerable number of times. When the test writing is repeated many times, however, this results in a situation where the processing load of the laser power adjustment is increased accordingly and a long period of time is needed for the laser power adjustment. Therefore, for commercialization, it is required to make it possible to accurately perform the laser power adjustment with around twice or three times of test writing.